run: do not lock machines if there are no roles

run: do not lock machines if there are no roles

The worker runs the job, even if there are no roles: it is legitimate to
run a job that does not need machines.

Run only locks machines if there are roles and init_tasks is only
populated with tasks that make sense depending on the presence of roles
or not.

Move the internal task added in suite.py to run.py : there is no reason
to not have them with teuthology-run or teuthology-schedule (ansible +
clock).

Signed-off-by: Loic Dachary <ldachary@redhat.com>